Impact of ARP lookup on local NTP traffic

[Gary E. Miller]

Yo Hal!

On Mon, 30 May 2016 17:34:09 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> gem at rellim.com said:
> > I have changed some "server" statements to "minpoll 4".  That
> > changes the NTP polling interval from 64 seconds to 32 seconds.
> > Since an ARP entry is elligible for eviction at 60 seconds that is
> > a nice cushion.   
> 
> Either one of us can't count or there is a typo in there.
> 
> 2 to the 4th is 16 rather than 32.

Whoops.  Thank you.  I did get it right in www/gpsd-time-service-howto.txt

> There is probably a kernel parameter you can tweak to make arp
> entries last longer.  I didn't find a simple answer with a quick
> search.  You can lock arp entries.  I haven't tried it recently.

Yes, but then that would only work for some version of Linux.  Changing
minpoll fixes all OS, and keeps the change local to avoid unintended
consequences.

This partly does it:

/proc/sys/net/ipv4/route/gc_timeout 

If you chnge it in the kernel you gotta change it for ipv4, and ipv6,

> If I was going to experiment with this, I'd use a pair of machines
> that weren't talking to each other (via NTP or whatever) and use ping
> and ip neigh or arp from the command line.

I have a simpler solution.  I;m running one NTP server with two
'peers'  One 'minpoll 5' and one default (minpoll 6).

ddrown already did some tests and has some nice graphs of the results.
I hope to have pretty graphs in a day ot two.
 
> I haven't found a simple way to print out the timer from the command
> line.

Which timer?  The poll times are in 'ntpq -p'

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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